This invention relates generally to induction machines, and, more specifically, to a control apparatus for regulating the frequency and magnitude of machine stator current or voltage in accordance with machine efficiency computed in real time to obtain maximum steady state machine efficiency irrespective of machine load conditions.
Adjustable speed induction machine drive systems have become increasingly popular for various industrial applications. Due to significantly increasing energy costs, it is highly desirable to operate these machine drive systems at peak efficiency. In large horsepower systems, a small increase in machine efficiency can result in substantial cost savings.
Prior art drive systems for induction motors fall into two primary categories: current-fed inverter and voltage-fed inverter drive systems. In both systems, machine performance is controlled by adjusting the magnitude and frequency of stator currents generated by the drive system and applied to the stator coils of the induction machine. In these systems, the air gap flux of the motor is either held constant or is varied in a predetermined functional relationship with developed torque. The later mode is designed to make the machine efficiency optimum under the assumptions that: (1) torque is uniquely related to speed; (2) machine parameters are constant; and (3) voltage and current waves do not contain any harmonics. In practical drive systems, such conditions can rarely be met.
In contrast, the control apparatus of the present invention obtains optimum machine efficiency by regulating the magnitude and frequency of machine stator current or voltage responsive to optimum machine air gap flux and optimum machine slip frequency which are determined in accordance with machine efficiency computed in real time.
It is an object of the present invention to provide a microcomputer based control apparatus for an induction machine drive system.
It is another object of the present invention to provide a microcomputer-based control apparatus for regulating frequency and magnitude of the stator currents (or stator voltages) of an induction machine drive system by feedback control responsive to optimum slip frequency and optimum machine air gap flux, respectively, which are determined in accordance with real time machine efficiency.